The Long, Long Reach of Atmospheric CO2 EnrichmentVolume 2, Number 16: 15 August 1999

In his comprehensive self-published book of ten years ago, our father wrote extensively about the many and diverse benefits of CO2-enhanced plant growth (Idso, 1989). One group of effects with which he was particularly enthralled occurred belowground and was described under the heading Soil Improvements.

"First and most obvious," he wrote, "is the stabilizing effect of enhanced plant cover on the world's valuable topsoil. Each and every year, billions of tons of this most important resource are lost to the ravages of wind and water. However, as plants grow ever more vigorously with increasing concentrations of atmospheric CO2, and as they subsequently expand their ranges to cover previously desolate and barren ground, both of these types of erosion should be significantly reduced, as has been demonstrated in numerous experiments with both natural and managed ecosystems."

Fast-forward to the present and the 12 August issue of Nature, where Rillig et al. (1999) also note that "soil aggregation is important for preventing soil loss through wind and water erosion." Their newest contribution to this subject of CO2-enhanced soil stability takes the benefits envisioned by our father and pushes them a full quantum leap forward; indeed, they identify an entire suite of totally new benefits, demonstrating that the consequences of atmospheric CO2 enrichment are lavishly distributed on both sides of the soil-air interface.

In a vast and multi-faceted research program carried out at the Jasper Ridge CO2 experiment site in northern California and the Sky Oaks CO2 experiment site in southern California, Matthias C. Rillig, Sara F. Wright, Michael F. Allen and Christopher B. Field studied belowground ecosystem responses to elevated atmospheric CO2 concentrations over a period of several years, focusing much of their attention on the growth responses of arbuscular mycorrhizal fungi that form symbiotic associations with plant roots. But they went much further than merely characterizing the growth responses of the fungi to the increases in atmospheric CO2 imposed in their studies; they quantified the fungal production of a glycoprotein called glomalin. And in an investigation of the consequences of this phenomenon, they examined its impact on the formation of small soil aggregates and the subsequent stability of those aggregates.

Rillig et al.'s reason for making so many detailed measurements was derived from the fact that the degree of soil aggregation and the stability of soil aggregates across many different soil types is closely related to the amount of glomalin in the soil. They wanted to see if the aboveground plant growth benefits of enriching the air with carbon dioxide would trickle down, so to speak, from (1) plant leaves to (2) plant roots to (3) symbiotic soil fungi to (4) fungal protein production to (5) soil aggregate formation to (6) the stability of the soil aggregates in the presence of water.

The scientists' hunch paid off. The amount of fungal-produced glomalin in the soils of the CO2-enriched treatments in all three of the ecosystems they studied was greater than that observed in the soils of corresponding ambient CO2 treatments. They also observed increases in the mass of small soil aggregates in the treatments exposed to elevated CO2; and the stability of the small soil aggregates in the CO2-enriched treatments was greater than the stability of the aggregates in the ambient CO2 treatments. In addition, in the Sky Oaks study, where six CO2 concentrations ranging from 250 to 750 ppm were imposed as treatments, the authors reported that "the proportion of soil mass in aggregates of 0.25-1 mm showed a linear increase along the CO2 gradient" and that "glomalin concentrations followed a pattern similar to that of the small aggregate size class," indicative of ever-increasing soil structure benefits with ever-increasing concentrations of atmospheric carbon dioxide.

The importance of these findings can hardly be overstated. Rillig et al. note, for example, that their results suggest that this phenomenon - the change in soil structure elicited by atmospheric CO2 enrichment - should be incorporated into global change research wherever possible, simply "because soil structure has a strong effect on soil processes and organisms." Indeed, the implications for erosion prevention alone are enormous. Considered together with soil improvements previously enumerated by our father, which are much more extensive than what we have mentioned, it is becoming ever more clear that the ongoing rise in the air's CO2 concentration is truly a blessing - in disguise to some, but obvious to others - even though hidden from view beneath the surface of the soil.